The present invention relates to a process for the solid state polycondensation of polyamide resins.
Polyamide resins, particularly PA6, PA6,6, PA11, PA12 and their copolymers, find wide application in both the fibres and flexible packaging area, and in articles produced by extrusion and blowing technologies.
While for fibres the relative viscosity of the resin is low (RV=2.4-3.0 for PA6) , for applications in the flexible packaging area and for the articles produced by extrusion/blowing technologies, higher relative viscosities are necessary (RV=3.2-50 for PA6).
The viscosity is increased to the required values (higher than 3) by means of SSP processes of the resin at temperatures between 140.degree. C. and 240.degree. C. in conformity with the type of polyamide used.
SSP processes are known for example from patent application EP-A-0 038 094, which describes a SSP process for polyamide 4, 6. In particular, the '094 application indicates a possibility of using phosphoric acid to accelerate the solid state reaction.
In U.S. Pat. No. 3,821,171, there is described a SSP process, wherein the chip is at first heated outside the reactor, and the heating phase is completed in the reactor. This heating can be made by using an inert gas, in particular nitrogen.
The SSP of 6 type polyamides was studied by M. R. Kulkarni and S. K. Gupta (Molecular Model for Solid State Polymerisation of Nylon 6.II. An improved model--Journal of Applied Polymer Science Vol. 53, 85-103 (1994)). On pages 95 and 96, it is indicated and reported in a graph, that the negative effect on the polycondensation kinetics of water concentration appears for concentration values higher than 0.001 mol/kg. In particular, the polymer reactivity is reduced, and the effect is evident for residence times higher than 6 hours. For this reason, it is necessary to reduce the water concentration in the vapour phase surrounding the solid phase by increasing the inert gas stream and/or reducing the vacuum level at which the present polycondensation reaction is carried out.
It results that by using a concentration of 0.01 mol/kg there is a reactivity reduction of about 15%.